A collection of riches

Thankfully, Fallout 3's official game guide, published by Prima Games shortly after the release of Fallout 3 and sold separately, provides us with an interesting hint about its origins.

The game guide's description of the mysterious stranger, Irvin, reads,

"You have a guardian angel, but with a deadly hand cannon instead of wings.

Meet the mysterious stranger. An odd and eldritch entity said to appear and aid you in combat before you draw your dying breath.

Of course, it helps to have the mysterious stranger first."